Microelectromechanical system cavity packaging

ABSTRACT

In described examples, a cavity is formed between a substrate and a cap. One or more access holes are formed through the cap for removing portions of a sacrificial layer from within the cavity. A cover is supported by the cap, where the cover is for occulting the one or more access holes along a perspective. An encapsulant seals the cavity, where the encapsulant encapsulates the cover and the one or more access holes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/846,190, filed Dec. 18, 2017, entitled “Microelectromechanical SystemCavity Packaging,” which application is hereby incorporated herein byreference in its entirety.

BACKGROUND

Electronic and electromechanical devices are often packaged in sealedenclosures. The sealed enclosures help protect such devices fromcontaminants, particles and moisture that might otherwise enter thepackage and mechanically or electrically disrupt normal operation of adevice. The potential effects of contamination generally become greateras increasingly smaller design features are used. Various MEMS(microelectromechanical system) devices include moving components andstructures (e.g., micro-miniaturized) that place unique demands on thesealed environment in which the MEMS devices are placed. The potentialfor contamination of the MEMS devices is often reduced by employingrelatively more expensive processes and materials for packaging a MEMSdevice.

SUMMARY

In described examples, a cavity is formed between a substrate and a cap.One or more access holes are formed through the cap for removing asacrificial layer from within the cavity. A cover is supported by thecap, where the cover is for occulting the one or more access holes alonga perspective. An encapsulant seals the cavity, where the encapsulantencapsulates the cover and the one or more access holes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a section view of devices mounted within a first sacrificiallayer on a substrate and covered by a cap in accordance with exampleembodiments.

FIG. 2 is a section view of access holes formed in the cap for exposingportions of the first sacrificial layer in accordance with exampleembodiments.

FIG. 3A is a top view of access hole covers formed over a secondsacrificial layer deposited over circular access holes in accordancewith example embodiments.

FIG. 3B is a section view of access hole covers formed over a secondsacrificial layer and for covering circular access holes in accordancewith example embodiments.

FIG. 4 is a section view of the substrate, the mounted devices, the capand the suspended coverings after removal of the first and secondsacrificial layers in accordance with example embodiments.

FIG. 5 is a section view of an anisotropic encapsulant layer for sealingthe access holes in the cap in accordance with example embodiments.

FIG. 6 is a flow diagram of a process for manufacturing devices,including encapsulated suspended access hole covers in accordance withexample embodiments.

FIG. 7A is a top view of access hole covers formed over a secondsacrificial layer deposited over rectangular access holes 232 inaccordance with example embodiments.

FIG. 7B is a section view of access hole covers formed over a secondsacrificial layer and for covering rectangular access holes inaccordance with example embodiments.

FIG. 8 is a top view of an integrated rectilinearly bridge-anchoredcover in accordance with example embodiments.

FIG. 9 is a top view of an integrated diagonally bridge-anchored coverin accordance with example embodiments.

FIG. 10 is a top view of an integrated radially bridge-anchored inaccordance with example embodiments.

DETAILED DESCRIPTION

In this description, the term “portion” can mean an entire portion or aportion that is less than the entire portion, and the term “encapsulant”can mean a substance that encapsulates and seals a covered portion of astructure.

Batch processing techniques are used to manufacture a wide variety ofdevices. Such devices include microelectromechanical systems (MEMS)devices. The MEMS devices are usually small and often include structuresformed at a micron scale. Such structures can include mechanicallymoving parts, which can be sensitive to heat buildup and contamination.Because of the potential for adverse effects of thermal build up andcontamination from gases, moisture and particles, MEMS devices are oftensealed for protection, for example. Accordingly, MEMS devices areusually placed on and/or formed on a substrate, such that the placedand/or formed MEMS devices are supported by the substrate and extendabove the substrate. The MEMS devices are protected by a sealedenclosure configured to shield MEMS components against potentiallyadverse environmental conditions and contaminants.

A significant amount of the cost of finished MEMS devices relates topackaging. For example, the cost of the MEMS devices is often increaseddue to the selection and use of relatively expensive materials andprocesses suited for protecting the MEMS devices. Such costs can includecosts of wafer bonding, custom singulation and die-level testing.Further, MEMS packaging often requires an interposer or sealing ring,which accounts for a significant amount of cost and footprint area of apackaged MEMS device.

In at least one example, a package is a substrate or a sealed container,which can include die, wafers and/or micromechanical devices in a localenvironment that is sealed from an outside environment. If MEMS devicesare not adversely affected by water molecules, then non-hermetic sealingcan be used. For example, packages can be sealed with non-hermeticsealants that include polymeric compounds. However, many MEMS devicesare sensitive to effects of water molecules, so the MEMS devices areaccordingly enclosed in packages that are hermetically sealed. Forexample, water-sensitive MEMS devices often include miniature movingparts that could undesirably adhere to adjacent surfaces as a result ofstatic friction (stiction) caused by water molecules.

Some wafer-level packaging techniques construct a “cage,” which includesa membrane with holes suspended above the MEMS structure. The membraneholes provide openings for an undercut (UC) agent to gain access insidethe cage to help dissolve and evacuate a temporary support structure,upon which the cage is formed. The temporary support structure has anundercut cavity. After the undercut cavity is formed, the membrane holesare sealed by coating the membrane holes with a liquid encapsulant,which helps provide a non-hermetic seal. The liquid encapsulant isselected such that the surface tension of the liquid encapsulant helpsprevent the liquid encapsulant against flowing inside the cage throughthe membrane holes. However, because of the encapsulant's chemicalnature, the encapsulant's constituent materials (e.g. compounds,molecules and atoms) could outgas into the cavity, which can affect theoperation of often-delicate MEMS structures.

Other wafer-level packaging techniques form undercut (UC) holes aroundthe periphery (e.g. outer edges) of a cage, through which an undercutagent is introduced to remove the temporary support structure. Forexample, the UC holes are formed around the periphery (e.g., as viewedfrom the top), such that an undercut process proceeds radially inwardsfrom the periphery to more central locations. However, the progressionof that undercut process is non-uniform, which often leads to irregularevacuation of the temporary support structure. The irregular progressionof the undercut process is worsened for larger cavities (and especiallywhen the UC holes are formed around the peripheral edges), which canextend the exposure time of some MEMS devices to the undercut agent andthereby potentially damage some MEMS devices.

A low-cost process for sealing devices (e.g., MEMS devices duringwafer-level packaging processes) is described herein. For example, atwo-dimensional array of holes is formed in the surface of a cap (seeFIG. 3A, for example) to provide access by an undercut agent for forminga cavity under the cap. A first set of holes is situated above aperipheral area of the cavity (e.g., to be formed), and a second set ofholes is situated above a central area of the cavity. The relativelyuniform distribution of the holes across the surface of the cap helpsensure a more predictably uniform progression of an undercut process forremoving a sacrificial layer beneath the cap. For example, the morepredictable rates of undercutting help protect MEMS devices fromundercutting agents and increase yields by limiting exposure times ofthe MEMS devices to the undercutting agents. The two-dimensional arrayof holes is covered by covers (supported by bridge anchors) thatrestrict an anisotropically formed encapsulate against entering theholes of the two-dimensional array.

FIG. 1 is a section view of devices mounted within a first sacrificiallayer on a substrate and covered by a cap in accordance with exampleembodiments. For example, the assembly 100 includes a substrate 110. Thesubstrate 110 is usually a portion of a wafer suitable for manufacturingintegrated circuits. For example, the substrate 110 can be a portion ofa silicon crystal or other such material suited for forming MEMSdevices.

The devices 120 are disposed (e.g., formed) on the substrate 110. Adevice 120 can be a MEMS device (such as an RF switch, a varactor ormicromirror), which is potentially susceptible to failure resulting fromcontaminants introduced in various stages in a manufacturing process.The devices 120 can be formed before and/or during the deposition of afirst sacrificial layer 122.

The first sacrificial layer 122 is deposited around (e.g., includingabove) the devices 120. The first sacrificial layer 122 can be formed bydepositing a photoresist in accordance with a photolithographic process(e.g., such that sloped edges of the photoresist are formed along theedges). The first sacrificial layer 122 protects the devices 120 andprovides a surface on which the cap 130 is formed. The first sacrificiallayer 122 is suited for removal by a removal process, which is selectedfor removing the first sacrificial layer 122 without harming componentsof the devices 120, such as an undercut process that includes plasmaetching or wet etching followed by a supercritical carbon dioxide dryingprocess.

The cap 130 is formed for sealing the first sacrificial layer 122. Forexample, the cap 130 is a metal layer formed above the first sacrificiallayer 122, such that the devices 120 and the first sacrificial layer 122can be hermetically sealed. The cap 130 can be a metal such as aluminum,a dielectric such as silicon dioxide, or a ceramic such as titaniumnitride. The cap 130 can be an inorganic material such as metal,dielectric or ceramic. The cap 130 can be transparent or opaque, or canhave various degrees thereof (in between transparent and opaque).

A metal or other conductive layer (not shown) are optionally depositedvia a physical vapor deposition (PVD) over the cap 130 (e.g., if the cap130 is not a metal) to help hermetically seal and/or electrically shieldthe devices 120 arranged beneath the cap 130. The PVD layer can be aninorganic material, such as metal, dielectric or ceramic. The PVD layercan be transparent or opaque, or can have various degrees thereof (inbetween transparent and opaque). The PVD layer is deposited beforeforming access holes, such as described hereinbelow with reference toFIG. 2.

FIG. 2 is a section view of access holes formed in the cap for exposingportions of the first sacrificial layer in accordance with exampleembodiments. The assembly 200 includes undercut (UC) access holes 232,each for providing access for a UC agent to pass through the cap 130.For example, the access holes 232 are formed in accordance with amask/reticle (not shown) for patterning a photoresist and etchingselected locations.

An etch process is performed to etch through and evacuate the selectedportions of the cap 130, such that access holes 232 are formed. The etchprocess can be a plasma (dry) etch or a wet etch in which the assembly200 is submerged in a liquid etchant for etching structures notprotected by a photoresist layer. The length of the etching process iscontrolled to control the depth of the etch, such that the access holes232 extend from a top surface of the cap 130 downwards and exposeportions of the first sacrificial layer 122.

In various embodiments, the access holes 232 can be one or more shapesof round (e.g., shown in top view in FIG. 3A), rectangular (e.g., shownin top view of FIG. 7), oval, elliptical, polygonal and polygonal withrounded corners. As described hereinbelow with reference to FIG. 3A, theaccess holes 232 are arranged in a two-dimensional pattern and/or gridto promote uniform etching of a first sacrificial layer 122 by UC agents(described hereinbelow).

FIG. 3A is a top view of access hole covers formed over a secondsacrificial layer deposited over circular access holes in accordancewith example embodiments. Assembly 300 includes a two-dimensional arrayof access holes 232 formed in the surface of the cap 130. A first set ofaccess holes 232 is formed in a peripheral area of the cap 130, and asecond set of access holes 232 is formed in a more central area of thecap 130 (e.g., between the optional one or more support posts 340). Therelatively uniform distribution of the holes across the surface of thecap 130 helps ensure a more predictably uniform progression of anundercut process for removing the first sacrificial layer 122 beneaththe cap 130.

After the access holes 232 are formed (such that the access holes 232extend through the cap 130 and partially into the first sacrificiallayer 122), a second sacrificial layer is deposited across the exposedstructures (including access holes 232 and the surface of the cap 130)for forming bridge spacers 334. The bridge spacers 334 are fortemporarily filling the access holes 232 and forming a planar surfacecovering one or more access holes 232. The bridge spacers 334 of thesecond sacrificial layer include an easily removed material, such as aphotoresist, for forming a planar surface above the cap 130. The bridgespacers 334 are disposed as a temporary support surface upon which thecovers 336 are formed.

In at least one example, the covers 336 are formed by depositing a metallayer for bridging a span of the bridge spacers 334. The deposited metallayer includes material for forming the cover 336 and is patterned andetched to selectively form the covers 336 (which are supported by one ormore bridge anchors 338 after the bridge spacers 334 are removed). Thecovers 336 are arranged in a two-dimensional pattern, such as in firstorientation arrays (e.g., rows) and in second orientation arrays (e.g.,columns).

Each cover 336 is a bridge-like structure (as shown in a section view ofFIG. 3B) for spanning a span “L” of the bridge spacers 334. Each cover336 includes bridge anchors 338 formed to rest on the cap 132 surface(e.g., instead of resting on the second sacrificial layer of the bridgespacers 334). The covers 336 span a width “W,” which is sufficientlywide for occulting an underlying access hole 232 structure (having adiameter “d” and centered in accordance with a pitch “P”). For example,the occultation exists in accordance with a directional perspective,such as a top view. Even after the access hole 232 is filled withmaterial of the second sacrificial layer 334, this description continuesreferring to it as the access hole 232.

Each cover 336 can be spaced from an adjacent cover 336 by an interval“G1,” which is sufficiently wide for forming a support post 340 betweenportions of the adjacent covers 336 (e.g., in a row). Support posts 340are formed using UC-resistant materials and are optionally formed tosupport central portions (e.g., instead of peripheral portions) of thecap 130, especially in designs where many devices 120 are to beprotected in a relatively large cavity 422 covered by a cap 130. Inrelatively large cavities, one or more access holes 232 (and covers 336)can be formed in the cap 130 between areas of the cap 130 supported byadjacent (including diagonally adjacent) support posts 340. Accordingly,one or more access holes can be formed within a first distance extendingfrom the one of the one or more access holes to one of the two supportposts, wherein the first distance is less than a second distanceextending between the two support posts.

Each support post 340 supports an area of the cap 130 extending at leasthalfway to an adjacent support structure, such as another support post340 or a vertically extending portion of the cap 130 that is formed onthe substrate 110. The support posts 340 can help prevent the capsurface against flexing inwards (and/or breaking a seal) in response toforces (e.g., atmospheric pressure) applied to the cap 130. The supportposts 340 can be an inorganic material such as metal, dielectric orceramic. The support posts 340 can be transparent or opaque, or can havevarious degrees thereof (in between transparent and opaque).

Each contiguous instance of the bridge spacers 334 underlies one or morerespective covers 336 and fills (e.g., plugs and/or planarizes) theaccess holes 232 to provide a planar surface for forming covers (e.g.,rows of covers) 336. Instances of the bridge spacers 334 include abreadth “L,” which is sufficient to span (and fill) the diameter “d” ofthe access holes 232. The distance between centers of the access holes(e.g., between centers of holes in rows, or in columns) can bedetermined based on etching rates and tolerance of included devices 120to the selected etchant. Area-based etching rates are describedhereinbelow with reference to FIG. 7A.

Accordingly, the access holes 232 are distributed across the top surfaceof the cap 130 in accordance with a first orientation (e.g., in rows)and a second orientation (e.g., in columns). The relatively evendistribution of the access hole promotes more uniform progression of anetching process, such as to more carefully control exposure time of adevice 120 to the selected etchant, and such as to increase yields.

FIG. 3B is a section view of access hole covers formed over a secondsacrificial layer and for covering circular access holes in accordancewith example embodiments. Assembly 300 includes a substrate 110, overwhich a first sacrificial layer (such as the sacrificial layer 122) isformed for encapsulating devices 120. A cap 130 is formed over the firstsacrificial layer and includes support posts 340, which are resistant toan etch (e.g., UC etching agent) for removing the first sacrificiallayer 122 in a subsequent process. One or more access holes 232 areformed through the cap 130 (which are openings for introducing a UCetching agent to the first sacrificial layer 122 in a subsequentprocess). A second sacrificial layer for forming a bridge spacer 334 isdeposited to blanket the access holes 232. The bridge spacer 334 is atemporary platform for forming the covers 336 (e.g., describedhereinabove with respect to FIG. 3A). The covers 336 are for occulting arespective access hole along a perspective, such as a top view.

FIG. 4 is a section view of the substrate, the mounted devices, the capand the suspended coverings after removal of the first and secondsacrificial layers in accordance with example embodiments. Assembly 400includes covers 336 arranged for occulting an open access hole (such asthe open access hole 232). After the covers 336 are formed (e.g., whereeach cover 336 is supported by bridge anchors 338 formed on the cap 130top surface), the first and second sacrificial layers are removed via anundercut (UC) process.

For example, the etching process begins by removing the directly exposedportions of the second sacrificial layer 334 and continues by etchingthe covered portions of the second sacrificial layer (e.g., a bridgespacer 334 underneath a respective cover 336) inwards toward theepicenter of a plugged access hole 232. The covers 336 remain supportedby the associated bridge anchors 338, such that an occulting portion ofeach cover 336 is suspended above a respective access hole 232.

After the epicenter of an access hole 232 is exposed to the UC etch,portions of the second sacrificial layer 334 within an associated accesshole 232 are removed by the etchant. Removal of the second sacrificiallayer 334 evacuates an associated access hole 232, such that the etchantgains access to the first sacrificial layer 122 via the distributed(e.g., centrally and peripherally located) pattern of access holes 232.

The etch process proceeds radially downwards and outwards from eachaccess hole 232 under the cap 130. The etch progresses such thatportions of the first sacrificial layer 122 subjacent to an access hole232 are etched at a uniform rate relative to the access hole 232. The UCetch process continues such that the sacrificial layer 122 is removed,and cavity 422 is formed. The devices 120 and support posts 340 areexposed within the cavity 422 formed by removal of the sacrificial layer122 between the cap 130 and the substrate 110. The uniform distributionof access holes, the etching rate, and the timing of the etching ratehelps ensure relatively uniform exposures times of various devices 120to the UC agent that is used to undercut the cap 130.

FIG. 5 is a section view of an anisotropic encapsulant layer for sealingthe access holes in the cap in accordance with example embodiments. Theencapsulant layer 540 is deposited over the covers 336 and the cap 130in accordance with an anisotropic process (e.g., in which properties ofthe processes are based on a direction of the application of a materialby the process). For example, the encapsulant layer 540 is an inorganiccompound selected and applied from an source above the assembly 500,such that the encapsulant layer 540 is deposited over uncovered surfaces(e.g., surfaces not covered by a cover 336 viewed from top view).Accordingly, the encapsulant is applied from a direction based on aperspective (e.g., top view) by which an access hole 232 is occulted(e.g., hidden from direct view) by a cover 336.

The anisotropically deposited encapsulant layer 540 forms a seal overexposed portions of the substrate 110, cap 130 and covers 336. Becauseof anisotropic properties, the encapsulant is limited against enteringan open access hole 232 and against potentially contaminating one ormore devices 120 via the open access hole 232.

In contrast, some encapsulation techniques rely on organic compounds,which rely on surface tension of the organic compounds to help impedeintrusion through an open access hole. However, outgassing of theorganic compounds can traverse the access hole (e.g., by process ofdiffusion) and contaminate underlying structures and devices. Further,such outgassing can be accelerated by heat applied to an organicencapsulant (e.g., where such heat is generated or absorbed duringoperation of the packaged devices).

FIG. 6 is a flow diagram of a process for manufacturing devices,including encapsulated suspended access hole covers in accordance withexample embodiments. Certain operations of flow 600 can be performed inan order other than a listed order. For example, structures of thedevices 120 can be arranged on the substrate 110 before or duringdeposition of the first sacrificial layer 122.

The flow 600 begins at operation 610, in which structures on a substrateare encapsulated and covered with a cap. For example, a firstsacrificial layer 122 is deposited around the devices 120 mounted on thesubstrate 110. The first sacrificial layer 122 can be formed bydepositing a photoresist (e.g., such that a slope is formed along theouter edges of the first sacrificial layer 122). The first sacrificiallayer 122 protects the devices 120 and provides a surface on which thecap 130 is formed. The cap 130 is formed for sealing the devices 120within the first sacrificial layer 122. The flow 600 continues tooperation 620.

At operation 620, access holes are formed by patterning and etching theaccess holes through the cap to the sacrificial layer. For example, aphotoresist is deposited over the surface of the cap 130, and thedeposited photoresist is exposed (e.g., with a reticle), such thatlocations for forming holes are patterned in the photoresist. A timedetch is performed to etch portions of the photoresist and cap 130 toform access holes 232. The duration of the etching process is controlledto control the depth of the etch, such that the holes 232 extend throughthe cap 130 and into a portion of the first sacrificial layer 122 (e.g.,for ensuring a complete etch of the selected portions of the cap 130).As described with reference to FIG. 3A and FIG. 7A, the access holes 232are arranged in a two-dimensional pattern and/or grid to promote uniformapplication and evacuation of UC agents. The flow 600 continues tooperation 630.

At operation 630, a cover is formed over a spacer to occult the accesshole. For example, a second sacrificial layer is deposited over theexposed access holes 232 and across a horizontal portion of the cap 130to form a planar spacer 334 covering one or more access holes 232. Theplanar spacer 334 is a temporary support surface upon which the covers336 are formed. In at least one example, the covers 336 are formed bydepositing a metal layer for bridging the spacer 334 and occulting (in afirst perspective such as a top view) the area defined by an associatedaccess hole. The flow 600 continues to operation 640.

At operation 640, the first and second sacrificial layers are removed.For example, an undercut process etches away: the second sacrificiallayer (e.g., beneath the covers 336 while leaving the covers 336 inplace); and the portion of the second sacrificial layer filling theaccess holes 232. After progressing through the access holes 232, theundercut process etches away the first sacrificial layer, such that acavity 422 is formed, and the devices 120 are no longer encapsulated bythe first sacrificial layer. The flow 600 continues to operation 650.

At operation 650, an encapsulant seals the covers, access holes and cap.For example, an encapsulant 540 is deposited in an anisotropic processto encapsulate the covers 336 and the top surface of the cap 130. Theanisotropic process deposits the encapsulant in accordance with adirectionality associated with the first perspective. The access holes232 are sealed because the encapsulant 540 is deposited to span thevertical voids between: the suspended edges of the covers 336; and thetop surface of the cap 130. The anisotropically deposited encapsulantspans the vertical void without intruding underneath (e.g., into a spaceocculted by) a cover 336, such that the encapsulant does not traversethe access holes 232 and does not contaminate the devices 120. In anembodiment, the anisotropically deposited encapsulant seals: all of theaccess holes 232; and any margin between the cap 130 and the substrate110. The encapsulant can be an inorganic or inorganic compound.

FIG. 7A is a top view of access hole covers formed over a secondsacrificial layer deposited over rectangular access holes 232 inaccordance with example embodiments. Assembly 700 includes atwo-dimensional array of access holes 732 formed in the surface of thecap 130. In at least one example, the access holes 732 are formed at asame height level as the surface of the cap 130. A first set of accessholes 732 is formed in a peripheral area of the cap 130, and a secondset of access holes 732 is formed in a more central area of the cap 130(e.g., between the optional one or more support posts 740). Therelatively uniform distribution of the holes across the surface of thecap 130 helps ensure a more predictably uniform progression of anundercut process for removing the first sacrificial layer 122 beneaththe cap 130.

After the access holes 732 are formed, a second sacrificial layer isdeposited across the exposed structures for forming bridge spacers 734.The bridge spacers 734 are for temporarily filling the access holes 732and forming a planar surface covering one or more access holes 732. Inat least one example, the covers 736 are formed by depositing a metallayer for bridging a span of the bridge spacers 734. The deposited metallayer includes material for forming the cover 736 and is patterned andetched to selectively form the covers 736.

Each cover 736 is a bridge-like structure for spanning a span “L” of thebridge spacers 734. Each cover 736 includes bridge anchors 738 formed torest on the cap 132 surface. Each cover 736 (including bridge anchors738) is spaced from other covers 736 in accordance with a pitch “P.” Thecovers 736 each are sufficiently wide to span a width “W,” which issufficiently wide for occulting an underlying access hole 732 structure(having a first length “d,” a second length “h” orthogonal to the firstwidth, and centered in accordance with a pitch “P”).

Each cover 736 can be spaced from an adjacent cover 736 by an interval“G₁,” which is sufficiently wide for forming a support post 740 betweenportions of the adjacent covers 736 along a first dimension (e.g., in arow). Each cover 736 can be spaced from an adjacent cover 736 by aninterval “G₂,” which is length of two bridge anchors 738 along a seconddimension (e.g., in a column). Support posts 740 are formed usingUC-resistant materials and are optionally formed to support centralportions of the cap 130.

Each contiguous instance of the bridge spacers 734 underlies one or morerespective covers 736 and fills the access holes 732 to provide a planarsurface for forming covers 736. Instances of the bridge spacers 734include a breadth “L,” which is sufficient to span (and fill) the width“d” of the access holes 732. The distance between centers of the accessholes (e.g., between centers of holes in rows, or in columns) can bedetermined based on etching rates and tolerance of included devices 120to the selected etchant. Further, the duration of the etching can bebased on the etching rate and the percentage of open area.

The percentage of open area for an embodiment can be determined inaccordance with Equation (1):

$\begin{matrix}{{Percentage}_{OpenArea} = {100 \times \frac{d - h}{P^{2}}}} & (1)\end{matrix}$where P>5. Different values for the Equations hereinabove can beselected in accordance with relative strengths of structures andetchants of various embodiments.

TABLE I shows example percentages of open area determined in accordancewith selected values of P, d, and h and Equation (1):

TABLE I P (in microns) d (in microns) h (in microns) % Area Covered 8 34 19% 12 7 8 39% 16 11 12 52% 20 15 16 60%

In accordance with the dimensions of TABLE I, reticles for forming thebridge spacers 734, bridge covers 736, bridge anchors 738 and supportpost 740 are useful to manufacture test devices, so etch rates (andconsistency of etch rates) are measurable to determine optimummaterials, processes and etch durations. The determined optimummaterials, processes and etch durations can be used in mass productionfor higher yield rates.

Accordingly, the access holes 732 are distributed across the top surfaceof the cap 130 in accordance with a first orientation (e.g., in rows)and a second orientation (e.g., in columns). The relatively evendistribution of the access hole promotes more uniform progression of anetching process, such as to more carefully control exposure time of adevice 120 to the selected etchant, and such as to increase yields.

FIG. 7B is a section view of access hole covers formed over a secondsacrificial layer and for covering rectangular access holes inaccordance with example embodiments. Assembly 700 includes a substrate110, over which a first sacrificial layer 122 is formed forencapsulating devices 120. A cap 130 is formed over the firstsacrificial layer 122 and includes support posts 740, which areresistant to an etch (e.g., UC etching agent) for removing the firstsacrificial layer 122 in a subsequent process. Through the cap 130, oneor more access holes 232 is/are formed as opening(s) for introducing aUC etching agent to the first sacrificial layer 122 in a subsequentprocess. A second sacrificial layer for forming a bridge spacer 734 isdeposited to blanket the access holes 732. The bridge spacer 734 is atemporary platform for forming the covers 736 (e.g., describedhereinabove with respect to FIG. 7A). The covers 736 are for occulting arespective access hole along a perspective, such as a top view.

FIG. 8 is a top view of an integrated rectilinearly bridge-anchoredcover in accordance with example embodiments. The top view 800 includesa rectangular cover 836 for occulting the access hole 232 in the cap130. The spacer 834 is a patterned second sacrificial layer 334. Thespacer 834 provides a planar surface upon which the cover 836 is formed.For example, the cover 836 is formed over the spacer 834, such that thebridge anchors 838 extend beyond the extent of the spacer 834 and rest(e.g., are anchored) on the top surface of the cap 130. The cover 836 isformed such that, when the spacer 834 is removed and the access hole 232is reopened, the access hole is occulted by the cover 834. Over the areapreviously occupied by the spacer 834, the cover 836 is supported by thebridge anchors 838 (e.g., which can be integrally formed with the cover836). The bridge anchors 838 are contiguous with the portion of thecover 836 that is overlying the space previously occupied by the spacer834.

FIG. 9 is a top view of an integrated diagonally bridge-anchored coverin accordance with example embodiments. The top view 900 includes asquare cover 936 for occulting the access hole 232 in the cap 130. Thespacer 934 is a patterned second sacrificial layer 334. The spacer 934provides a planar surface upon which the cover 936 is formed. Forexample, the cover 936 is formed over the spacer 934, such that diagonalbridge anchors 938 extend beyond the extent of the spacer 934 and rest(e.g., are anchored) on the top surface of the cap 130. The cover 936 isformed such that, when the spacer 934 is removed and the access hole 232is reopened, the access hole is occulted by the cover 934. Over the areapreviously occupied by the spacer 934, the cover 936 is supported by thediagonal bridge anchors 938 (e.g., which can be integrally formed withthe cover 936). The diagonal bridge anchors 938 are contiguous with theportion of the cover 936 that is overlying the space previously occupiedby the spacer 934. In an embodiment, a diagonal bridge anchor is formedat each of the corners of the square cover 936.

FIG. 10 is a top view of an integrated radially bridge-anchored inaccordance with example embodiments. The top view 1000 includes acircular cover 1036 for occulting the access hole 232 in the cap 130.The spacer 1034 is a patterned second sacrificial layer 334. The spacer1034 provides a planar surface upon which the cover 1036 is formed. Forexample, the cover 1036 is formed over the spacer 1034, such that radialbridge anchors 1038 extend beyond the extent of the spacer 1034 and rest(e.g., are anchored) on the top surface of the cap 130. The cover 1036is formed such that, when the spacer 1034 is removed and the access hole232 is reopened, the access hole is occulted by the cover 1036. Over thearea previously occupied by the spacer 1034, the cover 1036 is supportedby the radial bridge anchors 1038 (e.g., which can be integrally formedwith the cover 1036). The radial bridge anchors 1038 are contiguous withthe portion of the cover 1036 that is overlying the space previouslyoccupied by the spacer 1034. In various embodiments, radial bridgeanchor are formed around the circular cover 1036, such that the anglesubtended between any two radially adjacent bridge anchors 1038 is lessthan 180 degrees.

Accordingly, in described examples, one or more devices is/are mountedon a substrate and covered by a cap. In at least one example, the one ormore devices is/are encapsulated in a first sacrificial layer, overwhich the cap is formed. Access holes are formed through the cap toexpose portions of the first sacrificial layer. A second sacrificiallayer is deposited to fill and cover the access holes to form a spacer.A bridge structure is deposited over the spacer and patterned to form acover for occulting the situs of each access hole as viewed from a firstperspective. The first and second sacrificial layers are evacuated byetching the first and second sacrificial layer, which re-exposes theaccess holes. The access holes are sealed by depositing a contiguousanisotropic encapsulant over the suspended coverings and portions of thecap.

Modifications are possible in the described embodiments, and otherembodiments are possible, within the scope of the claims.

What is claimed is:
 1. A method of producing a device, the methodcomprising: forming a first layer over one or more devices; forming acap over the one or more devices encapsulated in the first layer;forming one or more access holes through the cap; forming a second layerover the one or more access holes; forming, on the second layer, one ormore covers over the one or more access holes; and removing the firstlayer and the second layer.
 2. The method of claim 1, further comprisingmounting the one or more devices on a substrate.
 3. The method of claim2, comprising: forming a first support post between the cap and thesubstrate; and forming a second support post between the cap and thesubstrate.
 4. The method of claim 3, a first distance between the firstsupport post and the second support post, a second distance between theone or more access holes and the first support post, wherein the seconddistance is less than the first distance.
 5. The method of claim 1,wherein the one or more devices comprises a microelectromechanicalsystem (MEMS) device.
 6. The method of claim 1, wherein removing thefirst layer and the second layer comprises removing the first layer andundercutting the second layer via the one or more access holes.
 7. Themethod of claim 1, wherein removing the first layer and the second layercomprises: plasma etching the second layer; and performing asupercritical carbon dioxide drying process.
 8. The method of claim 1,wherein removing the first layer and the second layer comprises: wetetching the second layer; and performing a supercritical carbon dioxidedrying process.
 9. The method of claim 1, further comprising depositingan encapsulant by an anisotropic process.
 10. The method of claim 9,wherein the encapsulant comprises an inorganic substance.
 11. The methodof claim 1, wherein the one or more access holes are in a two-dimensionarray of holes.
 12. The method of claim 1, wherein forming the firstlayer comprises depositing a photoresist.
 13. The method of claim 1,wherein the cap comprises metal, a dielectric material, or a ceramic.14. The method of claim 1, wherein forming the cap comprises depositingan inorganic layer by physical vapor deposition (PVD).
 15. The method ofclaim 1, wherein forming the one or more access holes comprises:patterning a photoresist layer to produce a patterned photoresist layer;and etching the one or more access holes in the cap in accordance withthe patterned photoresist layer.
 16. The method of claim 1, wherein theone or more access holes are round, rectangular, oval, elliptical,polygonal, or polygonal with rounded corners.
 17. The method of claim 1,wherein forming the one or more access holes comprises: forming a firstset of access holes in a peripheral area of the cap; and forming asecond set of access holes in a central area of the cap.
 18. The methodof claim 1, wherein the one or more covers comprises metal.
 19. Themethod of claim 1, wherein the cover occults the one or more accessholes.